Field of the Invention
The present invention relates to a fixing device which fixes an image on a recording medium, and an image forming apparatus including the fixing device.
Description of the Related Art
A variety of image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction machines combining several of the functions of these apparatuses, use a fixing device which includes a relatively thin fixing belt constructed, for example, of a metal substrate and an elastic rubber surface layer. With such a relatively thin fixing belt, the energy required to heat the fixing belt is substantially reduced, and a reduction in warm-up time and first-print time is achieved. The warm-up time refers to the time taken to raise the temperature of the fixing belt from a normal temperature to a predetermined reload temperature allowing printing when, for example, power is turned on. The first-print time refers to the time from the reception of a print request to the completion of a sheet discharging operation followed by a print preparatory operation and a printing operation.
As illustrated in FIG. 1, this type of fixing device includes an endless belt 100, a metal heat conductor 200, a heat source 300, and a pressure roller 400. The endless belt 100 serves as a fixing belt. The metal heat conductor 200 is formed into a pipe shape, and is disposed inside the endless belt 100. The heat source 300 is disposed inside the metal heat conductor 200. The pressure roller 400 is in contact with the metal heat conductor 200 via the endless belt 100 to form a nip portion N. In this case, the endless belt 100 is rotated by the rotation of the pressure roller 400. In this process, the metal heat conductor 200 guides the movement of the endless belt 100. Further, the endless belt 100 is heated, via the metal heat conductor 200, by the heat source 300 inside the metal heat conductor 200. Thereby, the entire endless belt 100 is heated. Accordingly, the first-print time following a heating standby time is reduced, and the shortage of heat in high-speed belt rotation is minimized.
To achieve further energy conservation and reduction in first-print time, the fixing device may be configured to directly heat the endless belt 100 without using the metal heat conductor 200. In the example illustrated in FIG. 2, the pipe-shaped metal heat conductor 200 is removed from the inside of the endless belt 100, and is replaced by a plate-shaped nip forming member 500 provided at a position facing the pressure roller 400. In this case, a portion of the endless belt 100 other than a portion of the endless belt 100 contacting the nip forming member 500 is directly heated by the heat source 300, thereby substantially improving the heat transfer efficiency and reducing power consumption. Accordingly, the first-print time following the heating standby time is further reduced, and moreover a reduction in cost due to the absence of the metal heat conductor 200 can be expected.
The fixing device may also be configured to include deformation preventing ribs for preventing the endless belt 100 from being pressed and deformed radially inward by, for example, a plurality of sheets fed in an overlapped manner.
In the fixing device including the above-described endless belt 100, at a position upstream of the nip portion N in the sheet feeding direction indicated by the arrows, the rotated endless belt 100 is pulled toward the nip portion N, and thereby tension is generated. In the configuration which guides the endless belt 100 by using the nip forming member 500, therefore, the rotated endless belt 100 comes into relatively hard contact with an upstream edge of the nip forming member 500, and thus may be damaged or broken.
Such damage or breakage of the endless belt is more likely to occur particularly in a fixing device which uses an endless belt further reduced in thickness to meet demand in recent years for energy conservation and reduction in first-print time and thus reduced in strength.